Properties of circulating fluidized beds (CFB) relevant to combustion and gasification systems

Circulating fluidized beds are popular for large-scale combustion operations, and they are also gaining attention for gasification of carbonaceous feedstocks. Circulating fluidized beds utilize the entire height of tall vessels for gas–solids contacting, with no distinct interface between bed and freeboard. The corresponding flow regime is usually fast fluidization for combustion and gasification of solid fuels. Operating conditions, riser geometry, hydrodynamics, heat transfer, solids feed and particle recirculation all have significant influence on the operation and effectiveness of the system. This chapter reviews some of the most important hydrodynamic properties of circulating fluidized beds that are relevant in CFB gas–solid combustion and gasification reactions, with particular attention to results for columns of rectangular cross section. Gas and solid mixing are summarized also, and a brief survey of relevant heat transfer aspects is provided.